I'd say I called it, but I never actually said anything of the sort anywhere online, so I guess I have no proof. Still, it was sort of easy to see coming, since Mawile already has a mega, and it just so happens to be a version exclusive in Ruby whose Sapphire counterpart didn't have one. So I guess it wouldn't have come close to topping the whole Doublade thing anyway.
My guess as to why Sableye didn't get a mega right off the bat is that it already was in a solid position with Prankster, whereas Mawile was just absolutely horrible, so it needed the hand more. Still, now justice is done and Sableye gets one too. And its flavor seems to be defense defense DEFENSE, judging by its appearance and lore. One can only wonder, though, what kind of ability would it get? Wonder Guard would be absolutely broken on a Pokémon with one fifth of Shedinja's weaknesses and 304 times the HP, so we can rule that out.
Other than that, though, are there any existing defense-based abilities that would justify using a Pokémon with a BST of 480, including base 50 HP, and no held item slot? I doubt it. Fortunately, patching is now possible, and it's already likely that X and Y will be getting a patch to get full compatibility with ORAS, considering this is by far the largest amount of new data introduced in the middle of a generation, and we're still over five months away from release. So the likelihood of a brand new ability is quite high. As for what it'll be... your guess is as good as mine. Maybe a Speed Boost variant that raises both defenses every turn instead of speed? It would fall in line with the lore, which mentions the gem's hardening process. Any other ideas?